Air Gapping, also known as air wall or disconnected network is a cybersecurity concept that refers to an individual device or network of devices that are not connected to the internet whatsoever.
This provides a way for network administrators to isolate a device from other devices, networks, or unsecured systems. Since there is a gap of air, the computer isn't directly connected to the network or devices, it can't be attacked through the network or other devices. So, to compromise a device, the hacker has to cross the roadblock (made of air) by physically sitting down in front of the device since there's no other way to gain access over a network.
These air-gapped devices are typically found in high-security environments such as military systems, air traffic systems, government and finance institutions, avionic systems and so on.